When replacing their windows, most DIY boat owners use plastic (either acrylic or polycarbonate). Plastic is easy to work with and produces seaworthy results. Plastic can be used to make curved windows; it can be fastened directly to the cabin trunk or fit into frames. There is much debate (often rancorous) about which is better: polycarbonate (i.e Lexan) or acrylic (i.e. Plexiglas). While we prefer polycarbonate for certain purposes, generally speaking we think that acrylic is better. In many cases, however, the best choice is actually tempered glass, but we'll have that discussion later.

Polycarbonate (i.e. Lexan)

DIY boat owners often get very excited about Lexan (which is the proprietary name for polycarbonate made by SABIC, formerly GE). Polycarbonate has gained a reputation for being virtually indestructible because it is used to make bullet-proof glass. Other characteristics of polycarbonate, however, are less appealing. Polycarbonate's strength is best understood in terms of impact resistance--polycarbonate doesn't break because it flexes. But this flexibility also makes it difficult to seal; polycarbonate windows are notorious for leaks. Sure, your Lexan may keep out bullets (and this may be important, depending on how you use your boat); but it may not be able to keep out the water. Polycarbonate is relatively soft, which helps it absorb a bullet’s energy but also means that it is easily scratched, even by salt water spray (which is much more prevalent at sea than flying lead). Friends of ours replaced the Lexan windows on their catamaran after only a couple of years of cruising because saltwater spray had rendered them almost opaque. Polycarbonate is also vulnerable to UV degradation. While you can purchase polycarbonate with a UV and abrasion resistant film, its effectiveness on a boat window is limited; curving the polycarbonate, moreoever, will compromise the film. Keep in mind, too, that polycarbonate is much more expensive than acrylic and, if you like a spiffy boat, needs to be replaced more often. That said, polycarbonate is excellent for storm windows, which don't need to be waterproof--just make sure that you leave enough room between the polycarbonate and the actual window so that when the polycarbonate flexes it does not break the window. Its is also a good idea to have a few pieces of polycarbonate on the boat for emergency repairs. On Momo we use a 10mm piece of polycarbonate for our companionway door. Given how much the material flexes, however, it would be a good idea to stiffen the polycarbonate with strips of teak--I don't think the polycarbonate will ever break, but I can imagine that if struck with enough force the door could blow out of the frame.

The question about the difference in strength between polycarbonate and acrylic needs to be kept in perspective. Unless you're sailing in a war zone, acrylic (often known by the proprietary name of Plexiglas) is strong enough for boat windows and hatches. Lewmar and Bomar use acrylic in their deck hatches; the Comex Deep Star tourist submarine uses acrylic for its pressurized hull and hasn't lost a tourist yet. Non-marine uses include protective glazing in hockey rinks, helicopter windows, and the walls and tunnels in large public aquariums. Other factors besides strength also play a role in determining which material is best suited for boat windows. Compared to polycarbonate, acrylic is less vulnerable to UV-light, resists scratches better, and is easier to seal. It is also less expensive. Momo's hatches (the largest of which is 30" x 32") are made of 3/8" acrylic, supported by crossbars. They are badly crazed after 30 years of relentless exposure to the sun, but they are very strong and don't leak. They are as firm under foot as any other part of the boat and an ideal place to set up the sewing machine when we need to repair a sail. The fact that we use our hatches as a convenient workspace has added a few scratches, but they would be much worse if the material was Lexan.

The plastic windows we've seen on blue water boats are usually between 5/16" and 3/8" thick. While plastic is easy to work with, it needs to be treated carefully. To prevent cracks, mounting holes should be substantially over-sized (i.e. a 1/4" fastener should have a 7/16" hole). Screws should not be countersunk; rather, their heads should rest flush on the surface of the plastic. Better yet, cushion the screws with a decent rubber washer. Avoid the use of black sealants because they absorb heat and exacerbate expansion and contradiction. When choosing sealants, avoid polyurethane adhesives like 3M 5200: the outgassing of the plastic will affect the seal and then you'll have windows that not only leak and but are also very difficult to take apart. Sika, 3M, and BoatLife all make products that supposedly work well with polycarbonate and acrylic. But it should be noted that polycarbonate and (to a lesser extent) acrylic are difficult to seal. Thus the directions for Sika's 295 UV (which is specifically meant for plastic windows) explicitly state that Sika "does not guarantee the performance of Sikaflex® 295 UV on acrylic, polycarbonate, or any other form of organic windows." Good results have been reported from silicone-based products used for glazing purposes in the commercial building industry, like GE's Silproof SCS 2000 and Dow 795. We've used Dow 795 successfully for many different projects (you won't find it at Home Depot; you'll have to hunt it down). One advantage is that it is easy to clean up and the residue is not as tenacious as regular silicone.

Thanks for a well written article giving the perspective of a user of Polycarbonate and Acrylic sheet. At HighLine Polycarbonate we produce Polycarbonate sheet and we have written a number of articles about Polycarbonate sheet properties and Polycarbonate vs Acrylic at www.highlinepc.blogspot.comWe have recently started looking at some technology to reduce the flexibility of Polycarbonate sheet, but keep its strength and clarity. Would this technology be something the boating community would be interested in? If there is interest we could potentially accelerate the development of the product. Please let us know - you can email us at info@highlinepc.com

Just had windscreen replaced and it feels like it's to short I'm sure the original screen tucked into the channel at the bottom behind the rubber but this one is sitting 5 mm above the channel and I can see the corner of the glass at the bottom,

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As a shipwright business located in the Caribbean we only use polycarbonate to replace hatch and portlight lenses. Due to the heavy and continuous sunlight we find that Acrylic crazes and breaks very quickly, and after seeing two people almost lose their legs when stepping on a plexi hatch that broke underfoot we highly recommend using polycarbonate if you are continuously in an extremely hot and sunny location.

Hi, love your post! I got away with it for a long time because I am always careful on boat work and fussy of finish but something I've found about working on boats. The window needs a relatively even surface. Besides the mechanical issues you don't want "waves" in your windows. thanks~ Bob Deleon

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My husband just purchased a used boat and the windshield needs to be replaced. I was thinking of getting a windshield installed for him for Christmas. I have been wondering what kind of windshield would be best. It seems like an acrylic windshield would work fine for our needs. Your description of the different types of plastic windshields is really explanatory. http://paplastics.com/boat-windshields/windshields/products.html

I think it is really essential to have a windshield on your boat. You can get going really fast, and it is just convenient to have a windshield. They are pretty easy to install, you just need to have the right tools. http://paplastics.com

I was surprised to see that the wildsereen was badly vandalized by what seemed to be a baseball bat and some piece of steel.I was happy to get the services of Novus and that he was able to replace the windows in no time

wind screen gold cost that like the next best thing if one were to choose a home away from home.The summer is just like the normal Malaysian weather but minus the sweltering heat, perhaps because of the relatively lower humidity here

Great information shared! Thanks for sharing this wonderful post I highly appreciate this and the information you provide it is very beneficial. about Lexan polycarbonate sheets.Good job done.Gold Coast Boat Sales

wind screen gold cost that like the next best thing if one were to choose a home away from home.The summer is just like the normal Malaysian weather but minus the sweltering heat, perhaps because of the relatively lower humidity here

I was surprised to see that the wildsereen was badly vandalized by what seemed to be a baseball bat and some piece of steel.I was happy to get the services of Novus and that he was able to replace the windows in no time

All windscreen repairs are designed to stop the damage from cracking further and regain as much of the original strength of your windscreen before the damage occurred.Your damage is subject to harsh road conditions and containments before being repaired such as rain and road grime which can effect optical clarity

Current windscreen is badly cracked and needs replacement. Any minor rust that might be hidden by the window rubber will also need to be repaired.If the windscreen is removed and bad rust is close,the wildsereen gold cost are here,

I just finished to install 3M 5200 around my lexan windows, and it leaks, we are on a sailboat in the Bahamas, it was the only type of sealant we could find around, like it was posted before the lexan did gas and the sealant did like a balloun

OUR TAKE ON THINGS

The last thing we want to do is encourage anyone to go sailing. There are far too many sailboats out here as it is; the ocean is getting crowded and the anchorages are packed. And with every boat out here, the rules multiply. It's dangerous, too: pirates, hurricanes, poverty, and inadequate healthcare! Let's keep those sailboats where they belong: firmly tethered to First-World docks, meticulous and clean, varnished by illegal immigrants to maintain resale value. As part of the effort to recover from the world's financial crisis, sailboats should be fitted with gear during the week and taken out on Sundays to produce the kind of head-turning envy that keeps the economy going. But nothing more than that. Long live the work-ethic! We' re here to tell you that there' s nothing romantic about stuffing a family of four into an oversized cigar box with spars and canvas. Nothing at all.

WHAT'S IN A NAME

From Michael Ende's Momo (1973):

Momo went over to the door, touched it with her hour-lily, which had only one petal left, and opened it wide. The time store was cold no longer, now that the last of the time-thieves had gone. Momo marveled at the contents of the huge vault. Innumerable hour-lilies were arrayed in its endless shelves like crystal goblets, no two alike and each more beautiful than the other. Hundreds of thousands, indeed, millions of hours were stored here, all of them stolen from people's lives. The temperature steadily rose until the vault was as hot as a greenhouse. Just as the last petal in Momo's hour-lily fluttered to the ground, all the other flowers left their shelves in clouds and swirled above her head. It was like a warm spring storm, but a storm made up of time released from captivity.